Existing projection television systems project an image from an electronic display device onto a large screen in a manner analogous to how a 35 mm slide projector projects a small slide onto a large screen. Display devices that are typically used in large screen TV and/or HDTV applications include a high brightness cathode ray tube (CRT), a Texas Instruments Digital Light Processor chip (DLP), a Liquid Crystal on Silicon chip (LCOS), or some other form of electronic display device. The display device can be reflective, transmissive, or self-emissive.
A key property of a CRT form of display device is that the image to be projected is “Lambertian.” When an image is Lambertian, the observed brightness of that image is independent of the viewing angle of the observer. For example, an image viewed on a CRT by a person should look equally bright from any angle, as on a computer monitor or a CRT-based TV set. The light is emitted uniformly into a hemisphere over 2 pi steradians, where a steradian is a unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere that is equal to the radius squared, such that the total solid angle of a sphere is 4 pi steradians. In order to capture and project as much of the light as possible, projection lenses for CRT-based projection systems must collect as large a solid angle as possible from the CRT. In other words, the lenses must be of a “high numerical aperture,” or correspondingly of a “low f/number,” where f/number (also known as f-number and f:number) is a measure of relative aperture of a lens, typically the ratio of focal length to the diameter of the exit pupil of the lens.
Since a CRT is Lambertian in light output, the solid angle or cone of light needs only to be sufficiently large for the desired screen brightness. The cone of light can be tilted or rotated with respect to the lens and CRT centerline.
A new class of CRT display devices uses Resonant Microcavity Phosphor (RMP) technology in order to direct most of the light emitted into a relatively small solid angle, or cone, normal to the surface of the CRT, as opposed to the Lambertian nature of traditional CRTs discussed above. Thus, instead of the light being emitted from the CRT phosphor in a Lambertian manner into 2 pi steradians (a full hemisphere) as with a conventional CRT, the light is emitted into a much smaller light cone with a centerline normal to the CRT surface. The net result of this is that brighter images with a better contrast will result on the screen.
There are several problems with current CRT projection lens technology. For example, existing CRT projection designs often use Fresnel lenses or “Liquid” lenses to redirect the light into the entrance pupil of the projection lens. The “liquid” lens is formed using a liquid contained between the external surface of the CRT faceplate and an optical element or “dome” in the lens design, for the redirecting element. This is often called a “C-element”. These lens approaches cause undesirable scattering, thermal sensitivity and stray light problems. An additional problem, in many projection lenses, is the complexity of the design that leads to high cost, temperature and/or tolerance sensitivity. Further, many projection lenses are designed for an internally curved CRT phosphor surface.